Imaging an exoplanet with a flower-power star shade

A specially designed shade can take a planet's host star out of the picture.

Late in May, at a session of the World Science Festival devoted to the prospects of finding life on other planets, astronomer Sara Seager came with a rather unusual looking stage prop—a thin black slab that tapered at one end. It sat off on the side of the stage for a while before Seager got the chance to explain what it was doing there. When she finally got the chance, she said why it could be the key to imaging small, rocky planets like Earth—and determining if their atmospheres provide hints that there might be life on the surface below.

So far, the only planets outside our solar system that we've imaged directly have been huge gas giants, far from their host star and young enough to still be glowing in the infrared. Even with their relative brightness and distance from the nearest star, the light from the star would completely swamp our sensors. So the telescopes used to contain what's called a coronagraph, or star shade. This blots out the central star, ensuring that the majority of light the telescope receives comes from the planets.

But small planets close enough to be in the habitable zone of a star create two problems. The first is that they don't produce any of their own light; instead, we'd have to capture light that's produced by the host star and then reflected off their surfaces or atmospheres. This makes them very dim, especially relative to their host star.

Seager said that the dimness itself isn't a problem, as nearby planets would be in the same range as the distant galaxies that have been imaged in the Hubble Deep Field observations (which are, in effect, extended exposures of a seemingly empty patch of sky). The contrast, however, does pose a problem. It's hard to build a coronagraph that can blot out a star without also blotting out its nearby planets—which is exactly what would happen if you stuck a coronagraph inside a ground-based telescope.

If you're imaging a distant star as a single point source of light, what you want is a coronagraph that's far enough away from the telescope that it, too, appears to be a point object. That means placing the coronagraph at a distance from the telescope. A distance of tens of thousands of kilometers, far enough that Seager and other people working on the project call the blocking device a star shade rather than a coronagraph. This dictates that the system has to be put in space and consist of two objects (the light-blocking shade and the telescope). These items must coordinate their locations very precisely despite a huge separation—then hold them for weeks as the telescope gathers the photons it needs to image faint, moving objects.

Seager's confident we can do that, but there was another problem with the idea: light is a wave. Its wave-like nature is what allows a lightbulb to send light down a hallway even when there's not a direct line of sight. In the same way, light from the star will start to bend around the star shade, creating an interference pattern of bright and dark bands. This is where Seager's stage prop comes in. Rather than a rounded disk, the star shade will look a bit like a flower, with petals radiating out from the central disk. If these are aligned just right (Seager mentioned millimeter-scale tolerances), these petals will create an interference pattern where all the bright areas are outside of the telescope. Against the dark background, any faint planets should be visible.

Right now, tests are underway at Princeton using a scale model of the star shade; these will be followed with tests using a larger model and bigger light source in the desert. If everyone's done the math right, this will validate the basic design.

That still leaves some significant questions. The first is whether the star shade, which will need to be folded up for its ride to space, can successfully unfold itself after its launch. As the video below shows, that's also being tested with a full-scale model. The second big question is whether NASA will fund it.

The star shade concept, along with some components being tested.

One thing that may complicate the funding situation is the fact that the alignment between the star, star shade, and telescope needs to be very precise, which means that the star shade will need to be moved around every time the imaging system switches targets. It therefore needs to carry a lot of fuel, and the system will have a lot of downtime while its parts are being relocated. One option would be to put more than one star shade into space, letting one move while the second is being used for observations. This would provide some insurance against problems in the unfolding process, but it would significantly up the cost.

If enough money is allocated, the project will be an obvious follow-up to the Transiting Exoplanet Survey Satellite, or TESS, which will identify planets orbiting nearby stars. That's set to launch in 2017, meaning this project might see first light early next decade.

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To clarify, the star shade (external occulter) is one of two different technology paths we're pursuing for direct imaging of exoplanets. The other is an internal coronagraph, which instead puts the starlight blocking capability inside the optics of the camera attached to the telescope. There are two parallel studies ongoing right now looking at "mid-scale" missions using either a star shade or a coronagraph (mid scale in this context means "not more than a billion bucks", btw.) The interim reports from the two study groups are available at http://exep.jpl.nasa.gov/stdt/ . Sara leads the star shade one, and the internal coronagraph one is led from NASA Goddard by Karl Stapelfeldt.

These are not exactly competing projects but rather 2 different paths to help increase the chance that one or the other pans out. Yes, the position keeping for the star shade is a formidable technical challenge (but one that smart people think can be achieved). Conversely the internal wavefront control and starlight suppression is an equally daunting challenge (but again one that smart people think can be achieved).

Now in the mean time there's a 3rd study going on as well. Remember a couple years ago when NASA was given some extra optics for unused Hubble-class spy satellites? The idea is to use one of those to implement the Wide-Field Infrared Survey Telescope mission that was the top ranked project for this decade, and (because a Hubble sized spacecraft is big enough for multiple instruments) to add an exoplanet imaging camera on there too. For various reasons this mission is planned to be in geosynchronous orbit, so the star shade doesn't work for the orbital dynamics. Thus this mission is pursuing the internal occulter approach. There's a lot of information on this one available at http://wfirst.gsfc.nasa.gov/add/. For comparison, on the one hand there are some complications from this not being a dedicated exoplanet-imaging-only mission -- but on the other hand it offers at least a 2x larger telescope to work with. The bottom line is nice too, since in this case we'd just be adding an exoplanet camera to an otherwise-already-existing mission, so the incremental cost is closer to $0.25B than $1B. (Of course the total mission cost will be more like $2B, but the point is it's doing way more than just exoplanet imaging. And at the end of the day you end up with a Hubble-quality telescope with 100x larger field of view than Hubble and a gigapixel imager…)

One option would be to put more than one star shade into space, letting one move while the second is being used for observations. This would provide some insurance against problems in the unfolding process, but it would significantly up the cost.

"First rule in government spending: why build one when you can have two at twice the price?" - SR Hadden - Contact

I think the article left out one additional complication. Both the star shade and the telescope will be in solar orbit. So they won't stay lined up on a given target without one or the other being actively moved (not just aimed) during an observation. Being in a solar orbit, the required movement will be small enough to be feasible, but it's definitely an added complication.

EDIT: This required movement is much greater than an adjustment for solar wind would require. You'll basically get solar wind adjustment "for free" while doing the much greater order of magnitude orbital mechanics adjustments.

The precise control sounds like a very interesting (that is, hard) problem. I imagine something like that would drift quite a lot from solar winds.

The different effects of n-body gravity would over time probably be an even bigger problem. While 10km isn't much distance at Solar system scales, it's enough that say at its closest approach to Mars, the effects of the encounter would shift the orbits of the two craft enough that a large amount of fuel would be consumed to realign them.

It's an important problem to solve though because, the same alignment techniques world be needed to make space based interferometer telescopes possible.

The precise control sounds like a very interesting (that is, hard) problem. I imagine something like that would drift quite a lot from solar winds.

The different effects of n-body gravity would over time probably be an even bigger problem. While 10km isn't much distance at Solar system scales, it's enough that say at its closest approach to Mars, the effects of the encounter would shift the orbits of the two craft enough that a large amount of fuel would be consumed to realign them.

It's an important problem to solve though because, the same alignment techniques world be needed to make space based interferometer telescopes possible.

Isn't interferometry a little different? I would have thought you could let the nodes (is that the right term) orbit freely. They all need to be aimed with precision, but they don't have to maintain a straight-line orientation the same way. Am I mistaken?

If I recall correctly, aren't some Lagrangian points "soft"? i.e. they are more regions, than points? If so, does anyone here know if any of the "soft" Lagrangian points in either the earth-moon, or earth-sun system are large enough to for the telescope and shade to operate in simultaneously, and thus limit fuel consumption?

"If enough money is allocated, the project will be an obvious follow-up to the Transiting Exoplanet Survey Satellite, or TESS, which will identify planets orbiting nearby stars. That's set to launch in 2017, meaning this project might see first light early next decade."

That's the thing though isn't it.

Our society just does not seem to value spending money on things that have a long term return, such as science, or increasingly it seems infrastructure.

I would rather we be building several of these rather than spending money elsewhere, such as on domestic surveillance.

To clarify, the star shade (external occulter) is one of two different technology paths we're pursuing for direct imaging of exoplanets. The other is an internal coronagraph, which instead puts the starlight blocking capability inside the optics of the camera attached to the telescope. There are two parallel studies ongoing right now looking at "mid-scale" missions using either a star shade or a coronagraph (mid scale in this context means "not more than a billion bucks", btw.) The interim reports from the two study groups are available at http://exep.jpl.nasa.gov/stdt/ . Sara leads the star shade one, and the internal coronagraph one is led from NASA Goddard by Karl Stapelfeldt.

These are not exactly competing projects but rather 2 different paths to help increase the chance that one or the other pans out. Yes, the position keeping for the star shade is a formidable technical challenge (but one that smart people think can be achieved). Conversely the internal wavefront control and starlight suppression is an equally daunting challenge (but again one that smart people think can be achieved).

Now in the mean time there's a 3rd study going on as well. Remember a couple years ago when NASA was given some extra optics for unused Hubble-class spy satellites? The idea is to use one of those to implement the Wide-Field Infrared Survey Telescope mission that was the top ranked project for this decade, and (because a Hubble sized spacecraft is big enough for multiple instruments) to add an exoplanet imaging camera on there too. For various reasons this mission is planned to be in geosynchronous orbit, so the star shade doesn't work for the orbital dynamics. Thus this mission is pursuing the internal occulter approach. There's a lot of information on this one available at http://wfirst.gsfc.nasa.gov/add/. For comparison, on the one hand there are some complications from this not being a dedicated exoplanet-imaging-only mission -- but on the other hand it offers at least a 2x larger telescope to work with. The bottom line is nice too, since in this case we'd just be adding an exoplanet camera to an otherwise-already-existing mission, so the incremental cost is closer to $0.25B than $1B. (Of course the total mission cost will be more like $2B, but the point is it's doing way more than just exoplanet imaging. And at the end of the day you end up with a Hubble-quality telescope with 100x larger field of view than Hubble and a gigapixel imager…)

To clarify, the star shade (external occulter) is one of two different technology paths we're pursuing for direct imaging of exoplanets. The other is an internal coronagraph, which instead puts the starlight blocking capability inside the optics of the camera attached to the telescope. There are two parallel studies ongoing right now looking at "mid-scale" missions using either a star shade or a coronagraph (mid scale in this context means "not more than a billion bucks", btw.) The interim reports from the two study groups are available at http://exep.jpl.nasa.gov/stdt/ . Sara leads the star shade one, and the internal coronagraph one is led from NASA Goddard by Karl Stapelfeldt.

was just going to ask if it was possible to do all this in the lens.

I know some security cameras already do this to avoid being blinded by lights shining on them.

It's an important problem to solve though because, the same alignment techniques world be needed to make space based interferometer telescopes possible.

Isn't interferometry a little different? I would have thought you could let the nodes (is that the right term) orbit freely. They all need to be aimed with precision, but they don't have to maintain a straight-line orientation the same way. Am I mistaken?

Nope, it's not that easy. You need to maintain the baseline between nodes of the interferometer to better than 1 wavelength of light (for whatever wavelength your interferometer is operating at), which means you do need active control of the baseline in order to maintain coherence.

If I recall correctly, aren't some Lagrangian points "soft"? i.e. they are more regions, than points? If so, does anyone here know if any of the "soft" Lagrangian points in either the earth-moon, or earth-sun system are large enough to for the telescope and shade to operate in simultaneously, and thus limit fuel consumption?

Sure, it's possible to put a star shade in a Lagrange point. Operation at Earth-Sun L2 has been extensively studied. But it doesn't help as much as you're suggesting. Precisely because these points are in practice not just points, spacecraft tend to drift in large local orbits around them. For instance the statement "JWST will operate at Earth-Sun L2" actually means "JWST will operate in an 800,000x200,000 km elliptical orbit in the general vicinity of Earth-Sun L2". It's a larger region than the orbit of the Moon. :-) So you can easily fit a star shade and telescope in there, but you'll still need to do relative stationkeeping as they both circulate around the Lagrange point.

Sorry if this is unrelated, but how practical or costly would it be to use this concept to shield Earth's polar regions from our sun?

Totally impractical, sorry. This is indeed just an updated version of Mr. Burns' sun-blocking device, with fancy edges to mitigate diffraction. :-) The shadowed region is not larger than the sunshade, so you'd need a sunshade larger than the polar regions to make any difference.

To clarify, the star shade (external occulter) is one of two different technology paths we're pursuing for direct imaging of exoplanets. The other is an internal coronagraph, which instead puts the starlight blocking capability inside the optics of the camera attached to the telescope. There are two parallel studies ongoing right now looking at "mid-scale" missions using either a star shade or a coronagraph (mid scale in this context means "not more than a billion bucks", btw.) The interim reports from the two study groups are available at http://exep.jpl.nasa.gov/stdt/ . Sara leads the star shade one, and the internal coronagraph one is led from NASA Goddard by Karl Stapelfeldt.

was just going to ask if it was possible to do all this in the lens.

I know some security cameras already do this to avoid being blinded by lights shining on them.

The hard part is that planets are really, really damn faint compared to stars. Like, part in ten billion faint for an Earth-sized planet in an Earth-sized orbit around a Sun-like star. So in order to be able to see one, you need to be able to block 99.999999999% of the light. (If you count you'll notice that's eleven 9s. No, I didn't miscount when typing. If you want more than 1-sigma confidence detection of your planet at 1e-10 contrast, you need your background down by a factor of 5 or 10 fainter than that…)

That is just painfully ridiculously hard, to say the least. There are days I think we're all nuts for thinking this is something we could actually do. At least it'll keep us busy for the next few decades. :-) If you want to do that level of blocking with an internal coronagraph you need absurdly good optics, picometer wavefront control, amazing suppression of diffracted starlight, and you almost certainly still have to do software post processing to enhance the contrast even more. Any system successfully implementing this will necessarily be very complex with many moving parts, interconnected control loops on multiple time scales, state of the art autonomous adaptive optics in space, etc. The star shade is attractive in part because it's elegant: all the suppression of starlight happens by free space diffraction. Conventional diffraction-limited Hubble-quality optics are more than adequate. Of course you've traded for a different set of complex problems with the deployment of a structure that needs an edge shape precise to microns over tens of meters, is basically a "34 meter razor-sharp throwing star in space" in the words of my colleague Aki Roberge, and has to do precision stationkeeping tens of thousands of km from the star.

It is, to say the least, less than obvious which of these really hard problems is going to be more tractable in the end. In fact there are some good arguments that the best approach would combine the two, using an internal coronagraph to rapidly survey and find planets (much faster switching between targets), and then only steering the star shade into position for the star systems where you already know for sure there is a detectable planet, to obtain deeper and higher contrast characterization observations over a broader wavelength range.

To clarify, the star shade (external occulter) is one of two different technology paths we're pursuing for direct imaging of exoplanets. The other is an internal coronagraph, which instead puts the starlight blocking capability inside the optics of the camera attached to the telescope. There are two parallel studies ongoing right now looking at "mid-scale" missions using either a star shade or a coronagraph (mid scale in this context means "not more than a billion bucks", btw.) The interim reports from the two study groups are available at http://exep.jpl.nasa.gov/stdt/ . Sara leads the star shade one, and the internal coronagraph one is led from NASA Goddard by Karl Stapelfeldt.

was just going to ask if it was possible to do all this in the lens.

I know some security cameras already do this to avoid being blinded by lights shining on them.

The hard part is that planets are really, really damn faint compared to stars. Like, part in ten billion faint for an Earth-sized planet in an Earth-sized orbit around a Sun-like star. So in order to be able to see one, you need to be able to block 99.999999999% of the light. (If you count you'll notice that's eleven 9s. No, I didn't miscount when typing. If you want more than 1-sigma confidence detection of your planet at 1e-10 contrast, you need your background down by a factor of 5 or 10 fainter than that…)

thanks for the extra detail.

I was taken in by the video's demonstration and thinking what was needed was on a much simpler level.

To clarify, the star shade (external occulter) is one of two different technology paths we're pursuing for direct imaging of exoplanets. The other is an internal coronagraph, which instead puts the starlight blocking capability inside the optics of the camera attached to the telescope. There are two parallel studies ongoing right now looking at "mid-scale" missions using either a star shade or a coronagraph (mid scale in this context means "not more than a billion bucks", btw.) The interim reports from the two study groups are available at http://exep.jpl.nasa.gov/stdt/ . Sara leads the star shade one, and the internal coronagraph one is led from NASA Goddard by Karl Stapelfeldt.

was just going to ask if it was possible to do all this in the lens.

I know some security cameras already do this to avoid being blinded by lights shining on them.

The hard part is that planets are really, really damn faint compared to stars. Like, part in ten billion faint for an Earth-sized planet in an Earth-sized orbit around a Sun-like star. So in order to be able to see one, you need to be able to block 99.999999999% of the light. (If you count you'll notice that's eleven 9s. No, I didn't miscount when typing. If you want more than 1-sigma confidence detection of your planet at 1e-10 contrast, you need your background down by a factor of 5 or 10 fainter than that…)

thanks for the extra detail.

I was taken in by the video's demonstration and thinking what was needed was on a much simpler level.

Like most everything involving space and astronomy, it's conceptually pretty simple, but implementing that concept to the precision needed is insanely complicated.

Its wave-like nature is what allows a lightbulb to send light down a hallway even when there's not a direct line of sight.

Uh, no. Reflective scattering allows a lightbulb to send light down a hallway, supplemented by a very small degree of refractive scattering. I'm not quibbling over your point that light can "bend" around objects because of it's wave-like nature, that's quite true, and is the basis for interferometry. But it doesn't work on the scale of a hallway in ordinary life, not for the materials in buildings and the wavelengths that we can observe with our own senses, and certainly not at the frequencies that you would get at any appreciable rate from a lightbulb.

So to sum, your physics is right, your scale is wrong, which makes your example wrong.

[edit] Grammer, be sure to see when you a word out, so the sentence sense. D'oh! (and that's in honor of the earlier Simpsons references)..

That is just painfully ridiculously hard, to say the least. There are days I think we're all nuts for thinking this is something we could actually do. At least it'll keep us busy for the next few decades. :-) If you want to do that level of blocking with an internal coronagraph you need absurdly good optics, picometer wavefront control, amazing suppression of diffracted starlight, and you almost certainly still have to do software post processing to enhance the contrast even more. Any system successfully implementing this will necessarily be very complex with many moving parts, interconnected control loops on multiple time scales, state of the art autonomous adaptive optics in space, etc. The star shade is attractive in part because it's elegant: all the suppression of starlight happens by free space diffraction. Conventional diffraction-limited Hubble-quality optics are more than adequate. Of course you've traded for a different set of complex problems with the deployment of a structure that needs an edge shape precise to microns over tens of meters, is basically a "34 meter razor-sharp throwing star in space" in the words of my colleague Aki Roberge, and has to do precision stationkeeping tens of thousands of km from the star.

So I guess you're looking at putting pretty precisely tuned lasers on the edges of the "star" to allow you do do some interferometry for the precise control position you'd need on the telescope? That should get you pretty close to the right tolerances, I would think. and the additional requirements for pointing the telescope for fine adjustments would probably be done easiest there, at the scope, with the gross adjustments obviously being required to be performed at the shade.

Although now that you've called it a throwing star, I can't get the image of a giant laser shooting throwing star out in space out of my head.

Very cool. Why is the backside reflective? My intuition would be to have a non-reflective backing.

I'd guess to help keep it cooler and reduce IR noise caused by a non-reflective back absorbing more of the suns light and getting hotter. Black body luminosity is proportional to t^4, so even a modest degree of extra cooling will significantly reduce the level of noise it's putting out.

Its wave-like nature is what allows a lightbulb to send light down a hallway even when there's not a direct line of sight.

That's not the reason. Light bounces around and is absorbed/re-emitted. If you have perfectly black surfaces and total vacuum, then that effect would be what you'd see, but it'd be tiny.

Yep. If it were due to the wave nature of light, you'd notice that the distribution of blue and red light was different, since red's wavelength is about twice blue's. Visible light's wavelengths are just too small compared to the lightbulb/shade/hallway dimensions to be noticed.

You can notice the difference between low-pitched sound's greater ability to go around corners than high-pitched sound. The wavelengths (e.g. 1 meter for 340 Hz sound) are comparable to the obstacles.

It will blow your mind. It too involves having a disc between a telescope and what it's looking at. However, it has nothing to do with blocking a sun or being a coronagraph. It's not particuarly for observing exoplanets -- it looks like it's for observing anything.

It's taking the light that comes in around the edges of the disc, like a Fresnel lens, and having it focus on the telescope mirror (many km or maybe thousands of km from the disc). It's supposed to dramtically increase the resolution of a telescope -- a lot of the headlines a few weeks ago were saying "1000 times the resolution of Hubble". It's incredible that blocking the view, in some sense, gives you a better view.